U.S. patent number 5,105,301 [Application Number 07/450,621] was granted by the patent office on 1992-04-14 for coupled quantum-well electro-optical modulator.
This patent grant is currently assigned to CSELT - Centro Studi e Laboratori Telecommunicazioni S.p.A.. Invention is credited to Domenico Campi.
United States Patent |
5,105,301 |
Campi |
April 14, 1992 |
Coupled quantum-well electro-optical modulator
Abstract
A coupled quantum well electro-optical modulator, whose
semiconductor structure is composed of one or of a plurality of
successions formed by two layers capable of forming quantum wells,
of a barrier layer which separates the two layers and of an
intrinsic layer which separates the succession from other possible
successions, where in presence of an electric field the absorption
threshold of the light radiation shifts towards shorter
wavelengths.
Inventors: |
Campi; Domenico (Turin,
IT) |
Assignee: |
CSELT - Centro Studi e Laboratori
Telecommunicazioni S.p.A. (Turin, IT)
|
Family
ID: |
11299149 |
Appl.
No.: |
07/450,621 |
Filed: |
December 14, 1989 |
Foreign Application Priority Data
|
|
|
|
|
Jan 26, 1989 [IT] |
|
|
67045 A/89 |
|
Current U.S.
Class: |
359/245; 257/17;
359/278; 257/E31.033; 359/248; 359/321 |
Current CPC
Class: |
B82Y
20/00 (20130101); H01L 31/035236 (20130101); G02F
1/01725 (20130101); G02F 1/01733 (20210101); G02F
1/0158 (20210101) |
Current International
Class: |
H01L
31/0352 (20060101); H01L 31/0248 (20060101); G02F
1/017 (20060101); G02F 1/01 (20060101); G02F
1/015 (20060101); G02F 001/03 (); G02F 001/23 ();
G02F 001/00 (); H01L 027/12 () |
Field of
Search: |
;350/355,356 ;357/3E,4
;359/245,248,278,321,322 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
D S. Chemla et al.; "Electroabsorption by Stark effect on room
temperature excitons in GaAs/GaAlAs multiple quantum well
structures"; Appl. Phys. Lett. 42(10) 15 May 1983 p. 864. .
D. A. B. Miller; "Novel Optical Modulators and Bistable Devices
using the Self Electro-optic Effect in Semiconductor Quantum
Wells"; The Second Int. Conference on Modulated
Semiconductors-collected papers Sep. 1985 p. 459..
|
Primary Examiner: LaRoche; Eugene R.
Assistant Examiner: Lester; Evelyn A.
Attorney, Agent or Firm: Dubno; Herbert
Claims
I claim:
1. A reverse-biased coupled quantum well electro-optical modulator
consisting of a structure composed of a plurality of semiconductor
layers, comprising:
a first metal layer allowing the structure to be connected to a
lead for the application of a control electrical potential,
a p-type layer in the form of an AlGaAs layer with a thickness of
about 1 micrometer below said first metal layer,
an intrinsic layer in the form of an AlGaAs layer having a
thickness of about 1 micrometer below said p-type layer,
at least one layer sandwich below said intrinsic layer and
comprising:
a first GaAs quantum well layer, and
an intrinsic AlGaAs barrier layer,
a second GaAs quantum well layer, and
a further intrinsic layer of the layer sandwich,
a first n-type layer in the form of an AlGaAs layer of a thickness
of about 3 micrometers below said further intrinsic layer,
an n-type cover layer below said fist n-type layer,
a substrate below said cover layer, and
a second metal layer allowing the structure to be connected to a
second lead of the application of the control electrical potential,
said metal layers, said substrate and said cover layer having holes
to allow the passage of a light radiation perpendicular to the
layers and entirely through all said layers and the modulator, said
first and second quantum well layers and said barrier layer, having
thicknesses in a range of 33.ANG..+-.10% so that said layer
sandwich presents a light radiation absorption threshold which
shifts toward shorter wavelengths in a blue shift when an
electrical field of the order of 10.sup.4 V/cm is applied, said
modulator absorbing little light radiation in an absence of said
electrical field.
2. The coupled quantum well electro-optical modulator defined in
claim 1 wherein all of said AlGaAs layers correspond substantially
to Al.sub.0.3 Ga.sub.0.7 As.
Description
FIELD OF THE INVENTION
The present invention relates to telecommunication using light
radiation as an information carrier and, more particularly, it to a
coupled quantum well electro-optical modulator.
BACKGROUND OF THE INVENTION
As is known, electro-optical modulators are of importance in
optical telecommunications systems. They, on the basis of an
electrical command, absorb or transmit optical pulses sent from a
suitable source, namely a laser, towards their surface. At their
output duly coded digital signals can be obtained to be transmitted
by optical fibers.
Present-day optical modulators exploit different physical effect,
e.g. magneto-optical, acousto-optical and electro-optical effects.
The latter effect, more particularly, is used in the so-called
"quantum-well" modulators. These devices consist of a succession of
layers of semiconductor material, a thin layer of which (having a
thickness lower than 250 .ANG.) forms the quantum well. The two
layers by which it is sandwiched, made of chemically-different
semiconductor materials, form the barrier layers.
This arrangement can not as an electro-optical modulator whenever
it is incorporated in an electric field perpendicular to the thin
layer. To this end the three layers are comprised between
semiconductor layers which in turn contact metal layers, to which a
suitable potential difference can be applied. The metal layers
allow the creation of an electric field, but do not influence the
physical phenomenon of absorption or transmission of light
radiation, dominated by the thin layer and by the confining barrier
layers.
The phenomenon, known as the Stark effect, is an energy level shift
induced by an electrical field in the thin layer material. In fact,
inside the quantum well there is a set of discrete and quantized
levels in the valence band as well as a set of levels in the
conduction band. Electron transitions from valence band to
conduction band take place by foton absorption with an energy
corresponding to an energy level difference. When an electric field
is applied, both in the valence band and in the conduction band an
energy level shift occurs and consequently, a shift in the light
absorption threshold of the device. More particularly, in the known
devices the absorption threshold shifts towards lower energy
levels, corresponding to longer wavelengths (red shift). Larger
shifts can be obtained by using highly-coupled multiple
quantum-well structures. Structures consisting of two quantum wells
separated by a very thin barrier layer, about 10A thick, are well
known (see "Electroabsorption in GaAs/A1GaAs coupled quantum well
waveguides", N.M. Islam et al, Applied Physic Lett. 50 (16), 20l
Apr. 1987). Absorption threshold shifts towards lower energies are
thus obtained which are about ten times greater than those
obtainable with a single quantum well. Also the applied electrical
field can be lower: about 8.10.sup.4 V/cm instead of 1.10.sup.5,
which is very near the breakdown voltage of the structure. However,
electro-optical modulators whose absorption threshold can be
shifted towards higher energy levels, corresponding to shorter
wavelengths (blue shift) have not previously been described.
OBJECTS OF THE INVENTION
It is the principal object of the present invention to provide an
improved coupled quantum-well electro-optical modulator which can
operate with reverse bias and which allows a shift of the light
absorption toward higher energy levels with application of an
electrical field, thereby increasing the fields of application of
such modulators.
Another object of the invention is to provide an improved
electro-optical modulator which avoids the drawbacks of the prior
art.
SUMMARY OF THE INVENTION
Basically the invention comprises between a p-type layer and an
n-type layer one or more layer sandwiches each of which includes a
layer forming the first quantum well, an intrinsic barrier layer
and a layer forming the second quantum well the layers of the
sandwiches having thicknesses such that the structure as a whole
has a light-aviation absorption threshold which shifts toward
shorter wavelengths when subjected to an electrical field provided
between metal layers on the p-type layer and on a substrate to
which the n-type layer is applied.
The coupled quantum-well electro-optical modulator, provided by the
present invention, allows a shift of the light absorption threshold
towards higher-energy levels, with application of an electric field
of the order of 10.sup.4 V/cm. This property offers greater
flexibility in optical telecommunications system design, since it
allows the designer to use the type of device more suited to the
optical radiation source available.
The present invention provides a coupled-quantum well
electro-optical modulator consisting of a structure composed of a
plurality of semiconductor layers, comprising,
a metal layer allowing the structure to be connected to a lead for
the application of a control electrical potential.
a p-type layer,
an intrinsic layer.
one or more successions comprising, a layer which forms the first
quantum well, an intrinsic barrier layer, a second layer which
forms the second quantum well, an intrinsic layer.
an n-type layer,
an n-type layer covering the substrate.
said substrate, and
a metal layer allowing the structure to be connected to a second
lead for the application of the control electrical potential, the
metal layers, the substrate covering layer and the substrate layer
itself having holes to allow the passage of a light radiation sent
perpendicular to the structure layers, characterized in that it
presents a light radiation absorption threshold which shifts
towards shorter wavelengths when it is subjected to the electrical
field.
BRIEF DESCRIPTION OF THE DRAWING
The foregoing and other characteristics of the present invention
will be made clearer by the following description of a preferred
embodiment thereof given by way of non-limiting example, and by the
annexed drawing illustrated in annexed drawing, the sole FIGURE of
which is a cross sectional view of the modulator structure.
SPECIFIC DESCRIPTION
Modulator performances and, more particularly, the shift of
absorption threshold of light radiation towards shorter
wavelengths, are obtained by starting from a structure with a
conventional layer succession, but using thicknesses both of the
thin layers forming quantum wells and of the barrier layer suitably
determined to allow blue shift described above.
The drawing shows a section of the coupled quantum-well modulator,
with a structure particularly adapted to operate on a light
radiation perpendicular to the layers. The thickness of the
individual layers is not shown to scale.
The uppermost layer 1 is a metal layer apt to permit structure
connection to a lead for the application of a control electrical
potential.
Below the layer 1 is a p-type A1GaAs layer 2, about 1 .mu.m thick,
acting as a contact layer.
Beneath the layer 2 is an intrinsic layer with a chemical
composition equal to that of layer 2 and with a thickness of about
1 .mu.m.
The layer below the layer 3 is the first quantum well, layer 4,
which is made of GaAs and is 33.ANG..+-.10% thick.
5 The layer 4 operates in a sandwich with a barrier layer 5, made
of intrinsic A1GaAs and 33.ANG..+-.10% thick and a layer 6 forming
the second quantum well, which is made of GaAs and is
33.ANG..+-.10% thick.
7 This sandwich is followed by an intrinsic layer 7, made of A1GaAs
and about 1 .mu.m thick. This layer, as well as layer 3, are
provided to avoid electrical field deformations due to interface
electrical fields induced by structure imperfections.
8 denotes an A1GaAs layer the aforedescribed stack is followed by
an AlGaAs layer 8; of n-type and about 1 .mu.m thick, a GaAs layer
9 of n-type and about 3 .mu.m thick, covering the commercial
substrate, which is generally imperfect, and the GaAs substrate
10.
Finally a metal layer 11 is applied to he substrate to allow the
structure to be connected to a second lead for application of the
control electrical potential.
To allow light radiation passage through layers g and 10 (which are
opaque at the operative wavelength) a hole is obtained by selective
chemical etching at 9 g, 10 g. Layer S blocks the etching. Of
course. even metal layers 1 and 11 have holes in correspondance
with the holes 1a, 11a in layers 9 and 10.
It is worth noting that layers 3 and 7 can be replaced by alternate
GaAs and A1GaAs layer successions, each being 10.ANG. thick, and
with a total thickness of about 0.5 .mu.m. Such a solution is very
expensive, but more efficient in removing undesired electrical
field deformations.
All A1GaAs layers present percent compositions equal to 0.3 in A1
and 0.7 in Ga.
As is to be noted, the structure is of p-i-n type (doped layer p,
intrinsic layer and doped layer n) and for its operation demands a
reverse bias voltage, capable of generating an electric field of
the order of 10.sup.4 V/cm.
With previously mentioned quantum well and barrier layer
thicknesses two effects are obtained upon creation of the
electrical field. A first effect still consists in the shift of the
first lowest energy transition towards lower energies (red shift).
but this transition becomes extremely inefficient, i.e. it absorbs
very little of the light radiation traversing the structure. The
second effect consists in the shift of the second transition of
higher energy than the preceding one, towards higher energies blue
shift) and in a high efficiency increase. In other words this
second transition absorbs very little light radiation when there is
no electrical field applied, while it produces an absorption 30 and
more times higher in the presence of the electrical field produced
by the reverse bias voltage.
Receiving a light radiation with a wavelength corresponding to the
first transition energy, the structure is highly absorbing with
null electrical-field. Upon application of the electrical field,
the first transition undergoes a red shift and becomes less
absorbing, while the second transition undergoes a blue shift and
becomes extremely absorbing, that is why the structure becomes
quasi -perfectly transparent at the light-source wavelength.
To obtain a cumulative effect it is possible to insert in the
structure a plurality of quantum wells, of the type of layers 4. 5.
G, separated by layers of intrinsic A1GaAs (such as layer 7) of at
least 100.ANG. thickness. Thus there is a succession of
coupled-well, 100.ANG. of A1GaAs, coupled well. 100.ANG. of
A1GaAs,. . . , coupled well. The total thickness of such a
succession can attain 0.5 .mu.m, corresponding to about 30 coupled
quantum wells.
The device can operate at a 10 Gbit/s modulation rate on radiation
with a wavelength of about 800 nm, whenever the material used in
the structure are those previously mentioned. Using different
materials, it is possible to operate on radiations with different
wavelength: e.g. by replacing A1GaAs with InA1As, GaAs with InGaAs
and GaAs substrate with InP substrate, the operative wavelength
attains 1,55 .mu.m. Since the effect of absorption threshold
towards higher energies (blue shift is determined exclusively by
the thickness of layer 5 forming the barrier, the material change
determines only a change in the wavelengths at which the
electro-optic modulator can operate, without perturbing the optical
blue shift effect. Finer adjustment of the operative wavelengths
can be obtained by changing thickness as of layers 4 and 6, forming
the quantum wells.
It is clear that what described has been given only by way of
non-limiting example. Variations and modifications are possible
without going out of the scope of the invention.
* * * * *